Detecting pickup



March 23, 1954 M. K. BAKER DETECTING PICKUP 2 Sheets-Sheet l Filed Feb. 16, 1950 March 23, 1954 M, K, BAKER 2,673,250

DETECTING PICKUP Filed Feb. 16,4 1950 2 Sheets-Sheet 2 Patented Mar. 232 1954 UNITED STATES PATENT OFFICE DETECTIN G PICKUP Application February 16, 1950, Serial No. 144,499

8 Claims.

This invention relates to a detecting pickup for use in electronic equipment.

The principal object of this invention is to provide an improved detecting pickup which readily responds to pressure Waves including those in the audio, supersonic and radio frequency ranges. which responds readily to the nature of objects adjacent thereto, which responds to movements and variations in the nature of objects adjacent thereto, which responds to electrical fields including those in the audio, supersonic and radio frequency ranges, and which responds to electrical elds set up by radio active substances. wherein all of such responses produce desirable electrical effects.

The detecting pickup of this invention preferably includes a pickup device and an oscillator associated therewith and controlled thereby. The pickup device includes a metallic plate having an oxide coating on one side, a metallic foil for engagfng the oxide coating and means such as a rubber jacket for resiliently pressing the metallic foil against the oxide coating. The metallic plate with the oxide coating is preferably provided with a large number of holes so that pressure waves may more readily vibrate the metallic foil with respect to the metallic plate and so that the pickup device may more readily generate external electrical fields. The oscillator may `be conventional in construction and may have the pickup device included in the tank circuit thereof for controlling the oscillator.

In the ycontrol of the oscillator it is believed that the pickup device acts as a variable capacity and a generator of external electrical fields, the

oxide coating acting as a dielectric between the metallic plate and the metallic foil.

Pressure waves, including those in the audio, supersonic and radio frequency ranges, are detected by the pickup device and operate to frequency modulate the oscillator. lIn this connection it is believed that the pressure Waves vibrate the metallic foil with respect to the metallic plate so that the pickup device operates primarily as a variable capacity for frequency modulating the L oscillator. The large number of holes in the rnen tallic plate appear greatly to increase the response of the pickup device to the pressure waves. Also, the use of the oxide coating as the dielectric appears further to increase the response of the picke up device to the pressure waves.

When objects are placed within the external eld, they affect the field in accordance with their nature. Various objects have different absorption and reflection characteristics and will correspondingly change the absorption and reflection of the generated eld. As a result the effective capacity of the pickup device is changed in accordance with the nature of the object and will operate correspondingly to change the frequency of the oscillator. The position of the object in the field will also affect the effective capacity of the pickup device and hence the frequency of the oscillator. If, therefore, the nature of an object in the external field and/or the position thereof in the external field varies from time to time, the effective capacity of the pickup device correspondingly varies and hence frequency modulates the oscillator in accordance with such variations.

The pickup device also responds to electrical fields set up by radio active substances and operates to change the frequency of the oscillator in accordance with such response. It is believed that such fields, including the alpha and beta rays, affect the pickup device to change the capacitative `tension thereof and hence the effective capacity thereof. Regardless of the possible theory involved, the detecting pickup of this invention does detect and respond quantitatively to the presence of radio active substances.

Likewise, the pickup device of this invention responds to electrical fields including those in the audio, supersonic and radio frequency ranges and operates to change the frequency of the oscillator in accordance with such response. Here again it is believed that these electrical fields affect the pickup device to change the capacitative tension thereof and hence the effective capacity thereof. Regardless of the possible theory here involved the detecting pickup of this invention does detect and respond quantitatively to the presence of such electrical elds including capacitative and magnetic fields.

It is also found by test and experiment that the Various responses of the pickup device of this invention are greatly enhanced by the use of the metallic oxide coating as the dielectric rather than other types of dielectric materials. The reasons for such improved responses are not entirely clear but it is believed that they result from ready electrical alignment of the molecules of the metallic oxide coating in accordance with the electrical elds affecting the same and from a rectifying action afforded by the metallic oxide coating.

In View of the wide Variety of conditions to which the detecting pickup and pickup device of this invention respond, it is apparent that they have a Wide variety of uses. As for example, they can be used as microphones, in communications applications, in instruments for detecting radio active substances and electrical elds, and in diagnosticinstruments.

Further objects of this invention reside in the details of construction of the detecting pickup of this invention and the cooperative relationshipv between the component parts thereof.

Other objects of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawings in which:

Fig. 1 is a sectional view through one form of the pickup device;

Fig. 2 is an elevational view of the active elements of Fig. Yl

Fig. 3 is a vertical sectional view similar to Fig. 1, lbut* illustratinganotherform of the pickup device;

Fig. 4 isa block .diagram illustrating adiagnostic instrument in which the pickup device of this invention may be utilized.

Fig. 5 is a wiring diagram .of the diagnostic instrument illustrated in Fig. Ll.

Referring first to Figs. l and 2 the pickup device is generally designated at i6. It includes a metallic plate Il which has formed on one side thereof a metallic oxide coating I2. For example, the metallic plate may be made of aluminum and may be substantially .025 inch thick. The metallic coating may be aluminum oxide substantially .0002 inch thick. The aluminum oxide may Vbe anodic aluminum oxide formed by anodic oxidation. Of course, other metals than aluminum may be utilized. For .example an oxidized titanium plate may be used. The .metallic plate and oxide coating are preferably provided with a plurality of holes I3.

A metallic foil Ill overlies and engages the metallic oxide coating I2 and has one end thereof secured to the oxide coating by suitable adhesive l5. The metallic foil is adapted to flex with respect to the metallic oxide coating` For example the metallic foil may be aluminum foil substantially .0001 inch thick but, of course, the foil could be made of other metals. Preferably the foil is made of the same metal as the plate in order to avoid electrolysis.

Means are provided for resiliently engaging the metallic foil It with the metallic oxide coating I2 and such means may include a rubberlike iacket i S which is stretched over and encompasses. the metallic plate II and the metallic foil i4. This rubber-like ,iacket may be formed of rubber latex or anv other suitable rubber-like material and since it is stretched it operates in tension for the purpose, of resilientlv engaging the metallic foil with the metallic oxide mating For example. latex. rubber substantially .001 inch thick in the free state may be stretched tn substantially .0005 inch thick for providing the desire, tension.

The metallic oxide coating I2 operates as a dielectric so that the metallic plate I i and metallic foil I4 form` a condenser, the capacitance cf which may be varied by flexing of the metallic foil. The capacitance of the condenser, of course, is determined by the construction thereof and the constrcticn may be varied in order to obtain desired capacitances for desired uses. ln the application of the pickup device to the diagnostic. instrumentV illustrated inFigs; 4 andV 5,

" oscillator.

4 the condenser preferably has a capacity of substantially twenty-live to fifty M. M. F.

Pressure Waves, including those in the audio, supersonic and radio frequency ranges, operate to flex the metallic foil for correspondingly controlling the capacity of the condenser. In this respect the plurality of holes vI3 inthe metallic plate I I provide for ready vibration-of the metallic foil in response to pressure waves. It is believed also that the metallic foil physically moves msympathy with alternating electrical fields affecting the same. Of course, instead of providing the plate with the metallic oxide coating, the foilcould be; provided with such coating for obtaining substantially the same results. The rubber-like jacket, in addition to providing resilient operation of the metallic foil, also effectively operates to protect the assembly from humidity, fumes, dust, dirt and the like.

When the pickup device I0 is located in the tank circuit Vof an oscillator, the metallic plate-I I, the metallic foil I4 and the. metallic oxide coat.-

ing I2 therebetween operate to generate anex-Y spends to the presence of radioactive substances and to the presence of electrical eldsincluding those in the audio, supersonic and radio .frequency ranges. For` example, when theV pickup device is included in the control or tank cir.- cuit of an oscillator, the frequency ofv the oscillator is varied by the presenceof radioactive substances suchA as radium paint on Vwatch faces and the like. It also responds toV concentrated electrical fields produced by external oscillators for regulating the frequency of the associated The reasons for such responses are not entirely clear but it is believed that the radioactive substances and electrical fields affect the capacitative tension and vhence the effective capacity between the metallic plate `II and the metallic foil I4. It is also believed they produce varying electrical alignment of the molecules of the metallic.- oxide coatingA I2 and that the metallic oxide coating affords a rectifying action. In any event, these responses are present and in an enhanced manner when the metallic oxide coating I2 is used as the dielectric material.

The condenser assembly is preferably provided with a metallic rim Il' spun thereon and in electrical contact therewith. The assembly is adapted to be contained within a housing I8 which in this instance is preferably made of suitable thermoplastic material. The housing I8 is provided with a probing face I9 having a substantially central opening 20 which is in alignment with the metallic foil I 4. The plastic housing I8 internally carries a metallic sleeve 2l which in turn frictionally receives a hard fibre washer The fibre washer engages the rim Il for holding the condenserassembly in place in the housing. S"rewthreadedly secured in the metallic sleeve 2I is a metallic disk 23 having an extension 24 forming part of a jack-plug.` A centra-l conductori having an external head 2l is receivedin the extension I24 and issuitably in-l sulated from the extension 24 by insulating material 25. The metallic rim Il is electrically connected by a conductor 23 to the sleeve 2| and hence to the extension 24. rThe metallic foil Hl is electrically connected by a conductor 29 to the central conductor and hence to the external head 2l. In this way the variable capacitance of the pickup device may be electrically connected to a suitable jack.

In the form of the pickup device illustrated generally at 3D in Fig. 3 the active elements H to I6 are substantially the same as those of Fig. 1, and like reference characters have been utilized for like parts. In Fig. 3 there is provided a metallic casing 3| having a shoulder 32 against which the condenser assembly abuts. IThe casing also has a rear face 33. A protecting disk 34 having an aperture 35 in alignment with the metallic foil I4 overles the Condenser assembly and in turn a wire screen 36 may overlie the disc 34. All of these elements may be secured in place by spinning over the edge of the casing 3l as illustrated at 31. The rear face of the casing 53 may be provided with an opening 33 in alignment with the opening 35. Electrical connection is provided between the metallic plate I I and the casinar 3l by reason of metal to metal contact therebetween. The rear face 33 is punched out to provide an opening therethrough with an eytension 33 for receiving an insulating plug dil. The insulating plug centrally carries a resilient contact strip M which resiliently engages the metallic foil I4 so that electrical connection with the metallic foil is thereby provided. The pickup device of Fig. 3 is particuf larly suitable for use in radio and communication applications.

For the purpose of illustratingr in detail one use of the detecting pickup of this invention, it is illustrated in Figs. e and 5 as incorporated in a diagnostic instrument. Referring particularly to Fig. 4, the detecting Pickup includes the pickup device I il and the radio frequency oscillator A designated at 45. The pickup device l0 regulates the frequency of the oscillator 45, the signal from which is amplified by a radio frequency amplifier 46. The ainplied signal then passes through a combination discriminator and detector t1 which produces a signal in accordance with the response of the pickup device If). This latter signal is amplified by an amnlier 48 and in turn operates an indicator 49, a meter 53 and/or a speaker 5l. When it is desired that the pickuo device H3 should respond to pressure waves such as soun-ds emanating from the body being diagnosed, the discriminator portion of the combined discriminator and detector ll-i is preferably used, and in this connection the pickup device I0 operates primarily to frequency modulate the oscillator 45. The discriminator detects the modulatine signal which in turn is ampliiied and operates the indicator, meter and speaker in accordance therewith. When the diagnostic instrument is utilized for the purpose of diagnosing conditions of body organs, such as the density thereof, then the detector portion of the combination discriminator and detector is preferably used. In this connection the density of the organs being diagnosed operates to change the effective capacity of the pickup device It which in turn operates to change the frequency of the oscillator 45. The oscillator d5 beats against a fixed radio frequency oscillator B designated at 52. The fixed frequency oscillator may be a crystal oscillator of conventional construction. The detector portion of the combined discriminator and detector detects the beat frequency, that is the difference between the frequencies of the two oscillators, and this beat frequency is utilized for operating the indicator, meter and speaker. The fixed frequency oscillator may be coupled into the diagnostic instrument at any suitable point ahead of the detector for producing the beat frequency. However, it has been found by test and experiment that much better results are obtained by shielding the fixed frequency oscilltaor 52 from the diagnostic instrument and providing the saine with an antenna 54 for generating an electrical field which permeates the body being diagnosed. It is believed that the pickup device lli in addition to being affected by the density of the organis being diagnosed is also affected by the radio frequency electrical fields generated by the xed oscillator and permeating the organs of the body.

Preferably, a Switch 53 is provided for rendering operative and inoperative the fixed frequency oscillator 52 and the discriminator and detector portions of the combination discriminator and detector il for adapting the diagnostic instrument for response to pressure waves or organ conditions. Even though the diagnostic instrument is switched so as to respond primarily to organ conditions, it also operates to detect and indicate pressures waves, this being accomplished by side-slope rectification in the detector portion of the combination discriminator and detector 41.

The details of the wiring of the diagnostic instrument are illustrated in Fig. 5, with the pickup device diagrammatically illustrated at I0. The radio frequency oscillator 45 includes an electronic tube 61') having a cathode 6I heated by a iilament E52, a control grid 83, a screen grid 64 and a plate 65. The control grid 63 is connected through a resistance 6G and a conductor to the source of a B+ potential, although it c may equally as well be connected to ground.

The screen grid 54 is connected through a condenser 58 to a ground conductor 69. One side of the filament 52 is connected to the ground conductor 59 and the other side to a conductor 'ill connected to A+ potential. The control grid es is connected through a condenser 'll to a control or tank circuit which includes the capacitance of the pickup device Ill and also a pair of variable inductances 'i2 and. 13. The cathode 5l is connected to the junction of the inductances 'l2 and i3. The plate 55 is connected through a variable inductance 'lll to the B+ supply conductor 63 and through the condenser 58 to ground. By suitably varying the inductance` of the indnctances l2 and i3 the frequency of oscillation of the oscillator l5 may be regulated. This may be accomplished by positioning a powdered iron core with respect to the inductances. For example, the frequency of the oscillator 45 may be regulated through afrequency range extending irom 1945 to 21.45 kc. and the fixed oscillator is tuned to substantially 2045 kc. Of course. other values of frequency and range of frequencies may be utilized.

Pressure waves, including those in the audio, supersonic and radio frequency ranges, which operate to Aflex the metallic foil I3 correspondingly vary the capacity of the pickup devices. rIhis in turn correspondingly varies the frequency of the oscillator and hence the4 cs oillator isl frequency modulated in response to such-,pressure waves. Since the pickup device is included in the control or tank circuit of the oscillator it generates an external electrical field, so: that when the probing face of the pickup devicei is placed against theA body, the generated electrical fieldis absorbed and reflected in accordance with Ythe density of the body at that point. This in turn operates to regulate the effeotive-capacity ofthe pickup device in accordance` with the density. Thus the pickup device operates to regulate the frequency of the oscillator. in accordance with the density. The plurality of holes in the metallic plate assist in thegenerating ofa relatively intense external electrical field and this external electrical eld is nlargely directedinto the body by the opening Z-llin the probing face of the pickup device.

vThe output'cf the oscillator 45 is transmitted through a link circuit including an inductance i associated with the inductance i4 and conn 'nectedby a pair of conductors 'I6 to an inductance l'i. Preferably, the oscillator 45 and pickup device lil are located remotely from the rest ofv theinstrument and connected thereto by a suitable cable including the conductors 6l, 69, 'lll` and le, The oscillator 45 is preferably included in asuitable small casing which may be readily manually manipulated, and this casing'and cable are` shielded by a grounded shield indicated at 18.`

VThe radio frequency amplifier Se may include a first stage having-an electronic tube provided with a plate 8|, a screen grid 32, a control grid 83, acathode dii and a filament @il and also a. secondistage having an electronic tube et pro- Y vided with a plate 3l', a screen grid E13, a control gridiiS, a cathode ecanda filament ei. The inductance l? of the link circuit is inductiveiy coupled to a variable inductance sii condenser 94 connected thereacross. The inductance. 93 is connected to the control grid 83 and to a conductor S5 which in turn connnuni- Cates .with ground through a condenser The cathode-.84 isconnected to ground. rThe plate Bi is connected to one end of a variable inductance Eil across which is connectedca condenserv 53. rlhe other end yof the variable inductance @l and thee-creen grid vS2 areconnected to the conductor` Inductively .f

19 leading from the B+ potential. coupled with the variable inductance Si is another variable inductance 99 which has a condenser i connected thereacross. The variable inductance is connected to the control grid 89, and to thefconductor S5. The cathode S53 is. connected to ground. The plate 8l is connected to a variable inductance ii having a condenser Iiii connected thereacross and the vari.- able inductance |62 and the screen grid 58 are connected to the conductor iii. Thus two stages of `radio frequency arnplication are provided.

The switch 53 for rendering operative and inoperative the discriminator portion and the detector. portion of the combined discrirninator and detector [il isV illustrated as a two position switch having a switch arm it for engaging contacts ill@ and H27, an overlapping switch arm m8 for engaging contact its and for engaging contacts It?. and Hal, a switch arm for engaeing contacts ||2 and H3, a switch arm H4 for engaging contacts ii and llt, a switch arm il'lfor engaging contacts H8 and H9, a switch arm liivfor engaging contacts |2| and |22, a switch arm |23 for engaging contacts |213 and |25 and a' switch arm |26 for'enga'ging contacts having a .L

|2-and |28. When the switch is in thetvlovver` positionillustrated, the discriminator portionisoperative and when in the upper position, the detector portion is operative. I

The combined discriminator andcdetector 47T will first bedescribed as a discriminatory. 'It includes a pair of serially arranged induetances i3d and |3| inductively coupled to the variable inductance |82. A condenser |32 is connected` across the inductances |33 and |3|. The junction of the inductances |3|i` and |3| is connected through a condenser |33,4 contact ISG, switch armiii and condenser |313 to the plate S1 of the radio frequency amplifier. Gne end of the inductance I3! is connected through switcharni |08 and contact its to one plate |36 of a double diodeV electronic tube |35. One end of the inductane l5!! is likewise connected through the switch arm; lili and contact ||5 to the other plateA |31-of the diode electronic tube. The diode tube iricludes cathodes |33 and |38 associated with the. plates |35 and |37. The cathode 38 is con-r, nected through switch arm i253 and `Contact |2|, serially arranged resistances Mil and and Contact itil and switch arm |23 to the cathode 39. Condensers |132 and M3 are connected across the resistances Ulti and 24|. The june-7 ture of the resistances hlt yand i4! is connected through acradiorfrequency choke ldclandcon-` tact H2 and switch arm |l| to the juncture of the inductances |39 and isi. The junctureof. the resistances |452 and lli is also connected through a .resistance M5,- contact H8. and switch arm il to the conductor which communicates with ground throughA the condenser` 96. The re-` sistance tit is connected through contact |21,- switch arni |26 and a condenser M6. to one end of a volume control potentiometer |4`i,the other end of which is connected to ground. The dis-f criminator portion is, therefore, conventional in construction and operation andoperates to produce a signal corresponding to the modulation of the oscillator Ii by the pickup device l5.

When the switch 53 is moved to the other position, the combined discriminator and detector operates as a detector. The switch |65 breaks the capacitative connection to the inductance` |3|. The switch |13 breaks the connection to the inductance |39. The switch It' connects the upper end of the inductance 3i lto the plates |3t and |31 in parallel. The switchll breaks the connection to the radio frequency choke and connects the lower end of the inductance 53| tol contact |28. The switch Hl breaks the connection to resistance hit and connects the lower endof the inductance |Si through resistance |49 to the conductor 95' which in turn com municates with ground through the condenser 96. The switches |23 and |23 connect the cathodes |33 and its in parallel to ground. Thus. the inductances |92 and Izi andthe' diode tube.` |35 operate as a conventional detector for detecting the difference in frequencies betweenthc oscillator e5 controlled by the pickup device and the fixed frequency oscillator 52 for producing a signal in accordance with such differences which is transmitted through the condenser W5" to the volume control potentiometer lli?.

The audio amplifier 48 preferably hastwo' stages of amplification. The Vfirst stage includes an electronic tube itt having a plate i5., a con" trol grid' |52, a cathode |53 and a filament |54. The second stage includes an electronic tube |551 havingV a platef |56, a screen grid |51; a control gridl; a cathode' 15S-and alame-nt' |69: The

cathode |53 of the rst stage is cdnnected to f ground through a grid leak bias consisting of a resistance |6| and a condenser |62. The control grid |52 is connected to the volume control potentiometer |41. The plate is connected through a resistance |53 to the conductor 1|] leading from the source of B+ potential and is also connected through a condenser |66 to the control grid |53 of the second stage. The control grid |58 is connected to ground through resistance It?. The cathode |59 is connected to ground through a grid leak bias consisting of a resistance |58 and a condenser |59. The screen grid |51 is connected to the conductor 10. The plate circuit of the second stage of amplication controls the indicator 49, the meter 59 and the speaker 5|.

The indicator 49 may include a lamp such as a neon lamp |19. The lamp |10 is connected to ground and through a resistance |1| to the plate |55. rlhe plate is also connected to the primary |12 of a transformer having a condenser |14 connected in parallel therewith. The primary |12 is connected to the source of B+ potential. The secondary |13 of the transformer is connected to the operating coil |15 for operating the speaker element |15 of the speaker 5|. The indicator 49 may be a milliameter |11 connected across the secondary |13 through a resistance |18 and a rectifier |19. The conductor 10 is connected through a resistance |64 to the source of B+ potential and through a condenser |65 to ground.

When the switch 53 is in the upper position and the fixed oscillator 52 is made operative the pickup device l0 is utilized primarily for responding to the density of organs adjacent thereto. The frequency of the oscillator 45 is regulated in accordance with the response of the pickup device lli to the organ density. And the oscillator 45 beats against the fixed oscillator 52. The beat frequency is detected by the detector portion of the combination discriminator and detector 41 for producing a signal for operating the indicator, meter and speaker in accordance with the beat frequency. Thus the speaker will have a characteristic tone for each density measured by the pickup device. By moving the pickup device along the body, tone changes will occur as the pickup device is moved over the edge of an organ. In this way, by listening to the tone changes the contour of the body organ may be readily determined. By the character of the tone the density of the organ and of portions of the organ may also be determined.

'Ihe audio amplier 48 is so arranged that an increase in beat frequency causes a decrease in the output voltage thereof. Since the indicator 49 and the meter 56 are operated in accordance with the output voltage of the audio amplifier, they will provide visual indications of the beat frequency and hence of the density of the body organ being diagnosed.

Pressure waves affecting the pickup device operate to frequency modulate the oscillator 45 in accordance with those pressure Waves and due to the nature of the detector the modulating frequency is also detected by means of side slope rectification. This modulating frequency caused by pressure Waves is audibly indicated by the speaker 5| and visually indicated by the indicator 49 and the meter 50.

The condenser 96 located between the conductor 95 and ground provides for maximum recovery rate in the electronic apparatus so that the electronic apparatus responds rapidly to changes in conditions affecting the pickup device.

When the swtich 53 is moved to the lower position to render operative the discriminator portion o1" the combined discriminatcr and detector 41, and the fixed oscillator 52 is rendered inoperative, then the pickup device Iii responds primarily to pressure Waves. Under these conditions the pressure waves affecting the pickup device Ill operate to frequency modulate the oscillator 45. The discriminator produces a signal in accordance with the modulating frequency i'or operating the speaker 5E and the indicator 49 and meter 50 for audibly and visually indicating the pressure Waves affecting the pickup device.

The pickup device |9 and the detecting pickup, consisting of the pickup device lil and the radio frequency oscillator 45, have many uses and applications other than in the diagnostic instrument described above. For example, they may be used as microphones in radio and communication applications and in instruments for detecting electrical fields and radioactive substances such as Geiger" counters and the like.

While for purposes of illustration two forms of this invention have been disclosed, other forms thereof may become apparent to those skilled in the art upon reference to this disclosure, and, therefore, this invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. A pickup device comprising a metallic plate, a metallic oxide coating on one side of the plate, a sheet of metallic foil for engaging the metallic oxide coating, and means for resiliently pressing the metallic foil against the metallic oxide coating.

2. A pickup device comprising a metallic plate having a plurality of holes therethrough, a metallic oxide coating on one side ci the plate, a sheet of metallic foil for engaging the metallic oxide coating, and means for resiliently pressing the metallic foil against the metallic oxide coating.

3. A pickup device comprising a metallic plate, a metallic oxide coating on one side of the plate, a sheet of metallic foil for engaging the metallic oxide coating, and a rubber-like jacket encompassing in tension the metallic plate and the metallic foil for resiliently pressing the metallic foil against the metallic oxide coating.

4. A pickup device comprising a metallic plate having a plurality of holes therethrough, a metallic oxide coating on one side of the plate, a sheet of metallic foil for engaging the metallic oxide coating, and a rubber-like jacket encompassing in tension the metallic plate and the metallic foil for resiliently pressing the metallic foil against the metallic oxide coating.

5. A pickup device comprising an aluminum plate, an anodic aluminum oxide coating on one side of the plate, a, sheet of aluminum foil for engaging the oxide coating, and means for resiliently pressing the aluminum foil against the oxide coating.

6. A pickup device comprising an aluminum plate, an anodic aluminum oxide coating on one side of the plate, a sheet of aluminum foil for engaging the oxide coating, and a rubber-like jacket encompassing in tension the aluminum plate and the aluminum foil for resiliently pressing the aluminum foil against the oxide coating.

7. A pickup device comprising a metallic plate, a metallic oxide coating on one side of the plate, a sheet of metallic foil of smaller dimension than the metallic plate for engaging the metallic oxide coating, meansffor resilientlypressing thelmetallic foilagainst' the "metallic oxide coating, "and a casing supportingand enclosing the metallic plate and metallic 'foil and `provdedwith an "opening in alignment with the metallic' foil.

"coating, means for 'resiliently pressing the metallic foil' against theme'tallic' oxide ooating,'and' a casing supporting'and enclosing the metallic plate and metallic' foil and provided adjacent thernetalllc platewith an'opening'in alignment with 15 themetallc foil. f

Referencescitea lin the me oflthis patent UNITEDrsTATEsir-ATENTS Number N umber-v Name Date Comstock May 11,1926 VWalle June 23, 1931 Bruno Sept. 20, 1938 Robinson Oct.'3,`1 939 Dubilier June 8, 1943 vKoch Nov. 12, 1946 Brewer July 27, 1948 FOREIGN PATENTS Country Date VGreat `l3ritain June 14.1.1934 Great Britain Nov. 28,11935 

1. A PICKUP DEVICE COMPRISING A METALLIC PLATE, A METALLIC OXIDE COATING ON ONE SIDE OF THE PLATE, A SHEET OF METALLIC FOIL FOR ENGAGING THE METALLIC OXIDE COATING, AND MEANS FOR RESILIENTLY PRESSING THE METALLIC FOIL AGAINST THE METALLIC OXIDE COATING. 